Cryoablation has been successfully used in various medical procedures to destroy or deactivate selected tissues. In this context, it has been determined that cryoablation procedures can be particularly effective for curing heart arrhythmias, such as atrial fibrillation. It is believed that at least one-third of all atrial fibrillations originate near the ostia of the pulmonary veins, and that the optimal treatment technique is to treat these focal areas through the creation of circumferential lesions around the ostia of these veins.
Typically, to cryoablate selected tissue in and around the heart in a non-invasive procedure, a cryo-catheter Is employed. In this regard, tissue in and around the heart is typically accessed from a peripheral artery such as the femoral or brachial artery. From the peripheral artery, the distal end of the catheter must navigate through the curves and bends of a narrow and tortuous vascular tree to reach a targeted area. In some cases, an introducer sheath is first inserted into the vasculature to establish a mechanical pathway to the treatment site. This allows the cryo-catheter to pass within the sheath from the peripheral artery to the treatment site. To be successful in locating the distal tip of a cryo-catheter at a treatment site, it is important that the catheter be flexible and have a relatively small outside diameter. On the other hand, modern cryo-catheters typically require the incorporation of a number of sophisticated, internal catheter systems that must all somehow fit within the thin, low profile catheter. These systems often include a first passageway to deliver a refrigerant from an extracorporeal location to the distal tip for expansion at the distal tip. A second passageway is also required to evacuate the expanded refrigerant from the tip.
In addition to the internal systems described above, various monitoring systems are often employed to measure tip temperature, tip pressure and electrical signals from the heart (i.e. EKG signals). These systems often require pressure tubes, wires, sensors, electrode bands and other monitoring components. Lastly, but perhaps equally important, modern cryo-catheters often include internal systems to articulate the distal tip of the catheter. These articulation systems can be used to steer the cryo-catheter during its journey through the vasculature and to manipulate the distal tip of the catheter into contact with selected tissue at a treatment site. For this purpose, these articulation systems typically include pull wires, sheath springs, deflection support structures such as springs, and other peripheral components. Thus, all of these system components need to somehow fit within a low profile cryo-catheter while still leaving sufficient room along the entire length of the catheter to deliver an ample quantity of refrigerant to the distal tip and evacuate expanded refrigerant from the tip.
With the above in mind, for a typical medical procedure, cryoablation begins at temperatures below approximately minus twenty degrees Centigrade (−20° C.). For the effective cryoablation of tissue, however, much colder temperatures are preferable. With this goal in mind, various fluid refrigerants (e.g. nitrous oxide N2O) have normal boiling point temperatures (i.e. the boiling point temperature at 1 atmosphere pressure) as low as minus eighty eight degrees Centigrade (−88° C). An important consideration in this regard is the fact that the temperature at which a refrigerant boils is dependant on the pressure that the refrigerant is experiencing. Specifically, for a refrigerant such as nitrous oxide, the boiling temperature increases with increases in boiling pressure.
A low ablation temperature, however, is typically not sufficient to efficiently cryoablate tissue. Specifically, it is also necessary that there is a sufficient refrigeration potential to effectively freeze tissue. In order for a system to both attain and maintain a suitable cryoablation temperature, while providing the necessary refrigeration potential to effect cryoablation of tissue, several physical factors need to be considered.
In this regard, it is well known that when a fluid boils (i.e. changes from a liquid state to a gaseous state) a significant amount of heat is transferred to the fluid from its surroundings. With this in mind, consider a liquid that is not boiling, but which is under a condition of pressure and temperature wherein effective evaporation of the liquid ceases. A liquid in such condition is commonly referred to as being “fully saturated.” It will then happen, as the pressure on the saturated liquid is reduced, the liquid tends to boil and extract heat from its surroundings. Initially, the heat that is transferred to the fluid is generally referred to as latent heat. More specifically, this latent heat is the heat that is required to change a fluid from a liquid to a gas, without any change in temperature. For some fluids, this latent heat transfer can be considerable. In this context, the refrigeration potential is a measure of the capacity of a system to extract energy from its surroundings at a fixed temperature.
An important consideration for the design of any refrigeration system is the fact that heat transfer is proportional to the difference in temperatures (ΔT) between the refrigerant and the body that is being cooled. Importantly, heat transfer is also proportional to the amount of surface area of the body being cooled (A) that is in contact with the refrigerant. In addition to the above considerations (i.e. ΔT and A); when the refrigerant is a fluid, the refrigeration potential of the refrigerant fluid is also a function of its mass flow rate. Specifically, the faster a heat-exchanging fluid refrigerant can be replaced (i.e. the higher its mass flow rate), the higher the refrigeration potential will be. This notion, however, has it limits.
As is well known, the mass flow rate of a fluid through a duct/tube results from a pressure differential on the fluid. More specifically, it can be shown that as a pressure differential starts to increase on a refrigerant fluid in a system, the resultant increase in the mass flow rate of the fluid will also increase the refrigeration potential of the system. This increased flow rate, however, creates additional increases in the return pressure (i.e. back pressure) that will result in a detrimental increase in the boiling point temperature of the refrigerant. Thus, for relatively low mass flow rates, increases in the mass flow rate of the refrigerant will cause lower temperatures. Refrigerant flow in this range is said to be “refrigeration limited.” On the other hand, for relatively high mass flow rates, increases in the mass flow rate actually cause the temperature of the refrigerant to rise. Flow in this range is said to be “surface area limited.” Because a cryo-catheter refrigeration system is least efficient at higher temperatures, operation under “refrigeration limited” conditions is generally avoided.
From the above discussion, it can be appreciated that a cryo-catheter refrigeration system must be capable of performing three basic functions. First, it must deliver the refrigerant to the distal tip of the cryo-catheter in a liquid state so that the liquid can boil at the tip and absorb latent heat. Second, the system must evacuate the expanded refrigerant and maintain the pressure where the refrigerant boils at a preselected pressure to ensure that the refrigerant boils at a low temperature. Lastly, the system must perform the first two functions at a sufficient refrigerant mass flow rate to generate the necessary refrigeration potential to efficiently cryoablate tissue. It is to be further appreciated that the satisfaction of these three requirements is highly dependent on the size of the flow passages and expansion chambers used to deliver the refrigerant to the cryo-catheter's distal tip and evacuate the expanded refrigerant from the tip.
In light of the above, it is an object of the present invention to provide a cryo-catheter configuration which optimizes both the catheter's outer diameter and the size of the catheter's internal refrigerant flow path. It is another object of the present invention to provide a cryo-catheter configuration that ensures that the cryo-catheter does not operate in a refrigerant limited condition. It is yet another object of the present invention to provide a configuration for a cryo-catheter that cooperates to maintain a refrigerant in a liquid state as it transits through a supply tube and simultaneously maintains the pressure in a refrigerant return line at about 1 atmosphere. Yet another object of the present invention is to provide a cryo-catheter configuration which is easy to assemble, relatively simple to implement, and comparatively cost effective.